1. Field of the Invention
The present invention relates to a signal reproducing apparatus and, more particularly, to a signal reproducing apparatus having the function of reproducing a frequency-modulated signal and equalizing the waveform thereof.
2. Description of the Related Art
A so-called cosine equalizer is used as a waveform equalizer which is provided in a reproducing part of a video tape recorder (VTR) which employs a conventional low-frequency-carrier frequency-modulated recording system.
FIG. 1 is a schematic block diagram showing the construction of the magnetic recording and reproducing system of a conventional VTR.
A modulating signal, such as a video signal, inputted through an input terminal It is frequency-modulated in a frequency modulator 1, then amplified in a recording amplifier 2, and then written to a magnetic tape 5 via a rotary transformer 3 by a rotary magnetic head 4.
During reproduction, a signal recorded on the magnetic tape 5 is read by a rotary magnetic head 6, then supplied to a reproducing amplifier 8 through a rotary transformer 7, and then amplified by the reproducing amplifier 8. The frequency characteristics of the thus-amplified signal are corrected for a magnetic reproducing part by a frequency-characteristic compensating circuit (FQ-CP) 9, and the losses suffered by the signal in the magnetic recording part are compensated for in a cosine equalizer 10. The resultant signal is frequency-demodulated in a frequency demodulator 12 and outputted through an output terminal Ot.
The FQ-CP 9 constitutes a transmission path which has the inverse characteristics of the resonance characteristics of a reproducing circuit consisting of the magnetic head 6, the rotary transformer 7, the reproducing amplifier 8 and so on. Accordingly, the output signal of the FQ-CP 9 is provided as a signal which has suffered losses only in the magnetic recording and reproducing system, with respect to the output signal of the frequency modulator 1.
The influence of the magnetic recording and reproducing system on the signal will be described below with reference to FIGS. 2 and 3.
FIG. 2 shows a spectrum distribution of a frequency-modulated signal outputted from the frequency modulator 1 of FIG. 1, and the shown example represents a spectrum distribution obtainable when a carrier frequency (j0) is 19 MHz; the frequency of a modulating signal, 12 MHz; and a deviation, 5 MHz. In each of FIGS. 2 and 3, j+1, j-1, j+2 and j-2 represent sideband spectra, respectively. When this frequency-modulated signal is recorded and reproduced, the spectrum distribution of the output signal of the FQ-CP 9 is as shown in FIG. 3. It is seen from FIG. 3 that an upper sideband is suppressed with a lower sideband emphasized as a result of the magnetic recording and reproducing process.
To compensate for such waveform distortion due to the magnetic recording and reproducing system, the cosine equalizer 10 of FIG. 1 is provided. The characteristic of the cosine equalizer 10 is shown in FIG. 4 and a conceptual diagram of its circuit, in FIG. 5.
The circuit shown in FIG. 5 includes a multiplier 13, current sources 14a and 14b, a delay line 15 (delay time .tau.), and a matching resistor 16 for the delay line 15. It is assumed here that an input signal ei of FIG. 5 is represented as: EQU ei=Ee.sup.j.omega.t (1)
Since only one side of the delay line 15 is terminated, the signal is reflected on the other side which is not terminated. Therefore, an output signal eo becomes: ##EQU1## and displays a characteristic such as that shown in FIG. 4 in the case of a=-1/3 or -1/6.
It is to be noted that the value of the coefficient a of the multiplier 13 which determines the characteristic of the cosine equalizer 10 is selected in advance so as to minimize the deterioration of image quality, by actually recording and reproducing various kinds of images.
However, the cosine equalizer of the above-described conventional example is unable to sufficiently compensate for losses suffered by a frequency-modulated signal of wide deviation in the magnetic recording part.
In a VTR which requires high image quality, such as a VTR for business use, the deviation is set wide to attain high S/N, so that the nonlinearity of the magnetic recording part appears noticeably. This will be described below with reference to FIGS. 6 and 7.
FIG. 6 is a graphic representation illustrating three spectra of the frequency-modulated signal having the spectrum distribution shown in FIG. 2, the carrier j0, a first lower sideband j-1 and a first upper sideband j+1, and the relative values of the three spectra with respect to the original carrier level are plotted for several deviations. The horizontal axis of FIG. 6 represents the deviations and shows that recording and reproduction with a maximum deviation of approximately 20 MHz have been carried out. In this case, the modulation degree exceeds 0.8, which indicates quite a wide deviation compared to a typical VTR.
FIG. 7 is a graphic representation in which three spectra of the output signal of the FQ-CP 9 obtained by recording and reproducing the signal of FIG. 6 are plotted in a manner similar to that of FIG. 6.
As can be seen from FIGS. 6 and 7, the first lower sideband j-1 is emphasized with the first upper sideband j+1 suppressed, and when the deviation is set wide, the carrier j0 itself is suppressed as well.
This phenomenon is caused by the nonlinearity of the magnetic recording and reproducing system and is unable to be completely compensated for by the cosine equalizer explained in connection with the conventional example.
An additional drawback of the above-described conventional example is that if the recording and reproducing characteristics vary due to variations with time, residual equalization error increases to deteriorate image quality. In particular, the variations of the magnetic recording and reproducing characteristics due to the wear of a magnetic head have been a great cause of the deterioration of image quality.